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The transition to reactor-relevant materials for the plasma facing components (PFCs) is an important and necessary step to provide a proof of principle that the stellarator concept can meet the requirements of a future fusion reactor by demonstrating high performance steady-state operation. As a first step to gain experience with tungsten as plasma-facing material in the Wendelstein 7-X (W7-X) stellarator, graphite tiles coated with an approximately 10 µm MedC tungsten layer (NILPRP Bucharest) were installed to complete the ECRH beam dump area in two of the five plasma vessel modules over an area of approximately one square meter each. In addition, tungsten baffle tiles are installed (40 tiles in total) with either bulk tungsten as part of NBI shine-through target or with a tungsten heavy alloy (W95-Ni3.5-Cu1.5) to replace the graphite tiles that were previously thermally overloaded. Based on an advanced diffusive field line tracing method and EMC3-Eirene simulations, the overloaded baffle tiles were redesigned by making the tiles thinner (i.e. moving the plasma-facing surface (PFS) away from the hot plasma region) and by reducing the local angle of incidence through toroidal displacement of the watershed. Significant erosion of the tungsten tiles can only be expected if sputtering by impurity ions such as carbon or oxygen ions contributes. However, the resulting central concentration of tungsten and the corresponding radiation losses are expected to be marginal. The expected deposition of carbon on the tungsten surfaces in the baffle regions mitigates further the possible tungsten enrichment in the core plasma. In OP2.1, no adverse effects of the installed tungsten PFCs on the plasma performance were observed during normal plasma operation. With the design changes made in the baffle area, the predicted heat load reductions could be experimentally confirmed.

The neoclassical ambipolarity condition governing the radial electric field in stellarators can have several solutions, and sudden transitions (in radius) between these can then take place. The radial position and structure of such a transition cannot be determined from local transport theory, and instead a non-rigorous model based on a diffusion equation for the electric field is usually employed for this purpose (Turkin et al 2011 Phys. Plasmas 18 022505). We compare global (full plasma volume) drift-kinetic simulations of neoclassical transport in the Wendelstein 7-X stellarator with this model and find significant discrepancies. The position r _0 of the transition is not predicted correctly by the diffusion model, but the radial structure of the transition layer is in reasonable agreement if the diffusion coefficient is chosen appropriately. In particular, it should depend on the plasma temperature in the same way as the plateau-regime coefficient of neoclassical transport theory or the gyro-Bohm diffusion coefficient. In the small-gyroradius limit, the prediction of r _0 by the diffusion model simplifies to the so-called Maxwell construction (Shaing 1984 Phys. Fluids 27 1567–9; Shaing 1984 Phys. Fluids 27 1924–6). However, this property also emerges from a wide range of other mathematical models in the appropriate limit. The basic assumption underlying these models is that the diffusion, or generalisations thereof, is independent of the radial electric field, which is however unlikely to be the case in practice. Presumably this fact explains the discrepancy between the diffusion model and the drift-kinetic simulations. Finally, it is found that global simulations replicate the phenomenon of spontaneous root transitions driven by variations in the electron-to-ion temperature ratio, as predicted by local theory in the small-gyroradius limit.

After a long device enhancement phase, scientific operation resumed in 2022. The main new device components are the water cooling of all plasma facing components and the new water-cooled high heat flux divertor units. Water cooling allowed for the first long-pulse operation campaign. A maximum discharge length of 8 min was achieved with a total heating energy of 1.3 GJ. Safe divertor operation was demonstrated in attached and detached mode. Stable detachment is readily achieved in some magnetic configurations but requires impurity seeding in configurations with small magnetic pitch angle within the edge islands. Progress was made in the characterization of transport mechanisms across edge magnetic islands: Measurement of the potential distribution and flow pattern reveals that the islands are associated with a strong poloidal drift, which leads to rapid convection of energy and particles from the last closed flux surface into the scrape-off layer. Using the upgraded plasma heating systems, advanced heating scenarios were developed, which provide improved energy confinement comparable to the scenario, in which the record triple product for stellarators was achieved in the previous operation campaign. However, a magnetic configuration-dependent critical heating power limit of the electron cyclotron resonance heating was observed. Exceeding the respective power limit leads to a degradation of the confinement.

The EMC3-Eirene code was used to study the main impurity leakage mechanism for the island divertor in the standard magnetic field configuration. It was found that under experimentally accessible plasma scenarios in the last experimental campaign, the majority of the island scrape-off layer was friction-force dominated. The impurity force balance was only thermal force dominated for upstream locations closed to the last closed flux surface, beyond the island X-point. No impurity neutral ionization was found in this location and hence the parallel impurity transport provides excellent impurity retention. It was found that impurities approach the confinement region nonetheless via perpendicular transport across the island O-point near the parallel flow stagnation region. This finding points out the specific role of the parallel flow stagnation region in providing lower parallel convective transport and long impurity residence times, which makes non-parallel transport channels more important or even the dominant driver of impurity leakage. In line with the relevance of the particle build-up in the flow stagnation region, different retention behavior as a function of density is seen for various species, which is shown to be due to ionization length changes as the plasma background density is increased.

The paper reports for the first time the heat and particle exhaust at the plasma boundary through various edge diagnostics for the high-performance plasma obtained after pellet injection on Wendelstein 7-X. The plasma density at the edge is found to be reduced by a factor of 2 in the high-performance phase, supporting the previously reported density peaking at the plasma centre. The plasma beta effect on the magnetic topology is reflected by the appearance of the second strike line, which is well understood with simulation. However, during the rapid decay phase of the enhanced confinement, a transient localized heat flow of up to 16 MW m ^−2 is observed at the leading edge of a poorly cooled divertor component, which has not been understood but raises concerns about machine safety.

In the Wendelstein 7-X (W7-X) stellarator, turbulence is the dominant transport mechanism in most discharges. This leads to a ‘clamping’ of ion temperature over a wide range of heating power, predominantly flat density profiles where hollow profiles driven by neoclassical thermo-diffusion would be expected and by rapid impurity transport in injection experiments. Significantly reduced turbulent transport is observed in the presence of strong core density gradients found transiently after core pellet injection and irregularly after boronisation or boron pellet injection. Density peaking is also achieved in a controlled manner in purely neutral beam heated discharges where particle transport analysis reveals an abrupt reduction in the main-ion particle flux leading to significant density profile peaking not explained by the NBI particle source alone. The plasmas exhibit a heat diffusivity of around $\chi = 0.25 \pm 0.1\,\mathrm{m}^2\ \mathrm{s}^{-1}$ at mid radius, a factor of around 4 lower than ECRH dominated discharges. Despite the improved confinement, the achieved ion temperature is limited by broader heat deposition and the lower power-per-particle given the higher density. This is overcome with limited reintroduction of ECRH power, where the low heat diffusivity diffusivity is maintained, the density rise supressed and ion temperatures above the clamping limit are achieved. The applicability of these plasmas for a high performance scenario on transport relevant time scales is assessed, including initial predictions for planned heating upgrades of W7-X, based on a range of assumptions about particle transport.

We present the first validated synthetic diagnostic for fast ion loss detectors (FILDs) in the Wendelstein 7-X (W7-X) stellarator. This model has been developed on, and validated against experimental data from, a FILD provided by the National Institute for Fusion Science (NIFS-FILD), with potential future applicability to the existing Faraday Cup FILD (FC-FILD) on W7-X as well as the scintillating FILD (S-FILD) currently under development. A workflow combining Monte Carlo codes BEAMS3D and ASCOT5 is used to track fast ions produced by neutral beam injection from the moment of ionization until they are thermalized or lost from the last closed flux surface, and from there to a virtual plane which serves as a projection of the entrance aperture to the FILD. Simulations in ASCOT5 are analyzed via a geometric method to determine the probability of transmission through the FILD aperture and onto the detector as a function of normalized momentum, pitch angle, gyrophase, and position at the virtual plane. This probability is then applied to the simulated ions arriving from the plasma, producing a simulated signal from a computationally tractable number of simulated fast ions. Simulated signals are presented for two W7-X experiments with neutral beam injection and quantitatively compared with experimental measurements from the NIFS-FILD diagnostic. An estimate of the frequency of charge-exchange with neutral particles in the edge is performed, and it is found that this process may have a significant impact on the measured signals.

We present the first nonlinear, gyrokinetic, radially global simulation of a discharge of the Wendelstein 7-X-like stellarator, including kinetic electrons, an equilibrium radial electric field, as well as electromagnetic and collisional effects. By comparison against flux-tube and full-flux-surface simulations, we assess the impact of the equilibrium ExB-flow and flow shear on the stabilisation of turbulence. In contrast to the existing literature, we further provide substantial evidence for the turbulent electron heat flux being driven by trapped-electron-mode and electron-temperature-gradient turbulence in the core of the plasma. The former manifests as a hybrid together with ion-temperature-gradient turbulence and is primarily driven by the finite electron temperature gradient, which has largely been neglected in nonlinear stellarator simulations presented in the existing literature.

Author(s): C. D. Beidler [sup.1] , H. M. Smith [sup.1] , A. Alonso [sup.2] , T. Andreeva [sup.1] , J. Baldzuhn [sup.1] , M. N. A. Beurskens [sup.1] , M. [...], Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak.sup.1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X).sup.2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator's non-turbulent 'neoclassical' energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas.sup.3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible.sup.1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization. Previously documented record values of the fusion triple product in the stellarator Wendelstein 7-X are shown to be evidence for reduced neoclassical energy transport in this optimized device.

In the published paper titled ‘Gas exhaust in the Wendelstein 7-X stellarator during the first divertor operation’ (2022 Nucl. Fusion https://doi.org/10.1088/1741-4326/ac7ac4 ), figure 8 was incorrect. This corrigendum provides the correct figure 8 .

In the first divertor campaign in Wendelstein 7-X (W7-X), unexpected significant heat loads were observed at particular plasma-facing components (e.g. baffle tiles and middle divertor part) which were not designed to receive high heat flux. In a prior investigation, it was concluded that the previous diffusive field line tracing (DFLT) model used for divertor design in W7-X cannot reproduce these loads, due to the missing physics in simulating the heat transport in the shaded flux tubes. To tackle this issue, two new efficient codes (DFLT_rev and EMC3-Lite) are introduced and validated against various experimental heat distributions in different magnetic configurations. The new tungsten baffle tiles have been designed with these codes and mounted in the machine, aiming for mitigated heat loads in the upcoming campaign.

Drifts affect particle, momentum, and energy transport in the scrape-off layer (SOL) of tokamaks and stellarators, altering plasma flows and creating asymmetries between divertors. To understand how drifts affect SOL transport in the W7-X island divertor, an experiment was performed to compare plasmas with matched core parameters but opposite magnetic field directions, and therefore opposite drift transport directions. Parallel flow measurements made with coherence imaging spectroscopy are interpreted with the aid of a diagnostic forward model and a 1D simple SOL model that includes the $\boldsymbol{\mathbf{E}} \times \boldsymbol{\mathbf{B}}$ drift. In low-density plasmas ( $\overline{n}_\mathrm{e} \lt 2 \times 10^{19}$ m ^−3 ), the poloidal $\boldsymbol{\mathbf{E}} \times \boldsymbol{\mathbf{B}}$ drift induces a large poloidal density asymmetry within the island SOL, as measured by divertor Langmuir probes. This in turn causes the parallel flow stagnation point to shift from the position halfway between targets to the X-point in the drift direction, leading to near-unidirectional flow throughout the SOL. As density increases, the effects of the poloidal $\boldsymbol{\mathbf{E}} \times \boldsymbol{\mathbf{B}}$ drift decrease substantially, resulting in a smaller density asymmetry and the development of a counter-streaming flow pattern. For the entire density range probed in this experiment ( $\overline{n}_\mathrm{e} = 1.5-6\times 10^{19}$ m ^−3 ), the experimental observations are more consistent with the effects of the poloidal $\boldsymbol{\mathbf{E}} \times \boldsymbol{\mathbf{B}}$ drift than the radial $\boldsymbol{\mathbf{E}} \times \boldsymbol{\mathbf{B}}$ drift.

Background: Labor outcome with regional anesthesia following previous cesarean section has been fraught with concerns regarding uterine rupture. There is sparse literature regarding the association between the impact of epidural analgesia and labor outcome in vaginal birth after cesarean section (VBAC). This study aims to evaluate the effect of labor epidural analgesia on labor outcome following TOL (Trial of labor) in previous cesarean or normal delivery with maternal and neonatal outcomes. Material and Methods: This is a prospective controlled longitudinal study in second-gravida patients in labor. A total of 132 patients were enrolled for the study out of which 101 were divided into three groups. Group A included 38 second-gravid women with a history of previous caesarean delivery and Group B included 32 second-gravid having previous normal vaginal delivery, both the groups received epidural analgesia during labor. Group C included 31 second-gravid with a history of previous cesarean section who did not receive epidural analgesia during labor. The aim of the study was to evaluate labor outcome in terms of successful vaginal delivery with or without epidural analgesia, along with visual analogue scale (VAS) pain scores during the conduction of delivery, hemodynamic parameters, and progress of trial of labor (TOL). Other obstetric and neonatal parameters were also evaluated. Results: Vaginal delivery was conducted in all patients in Group B (32/32; 100%) and Group C (31/31; 100%), whereas in Group A, two patients underwent emergency cesarean section as a result of impending uterine rupture in one case and acute fetal hypoxia in another. According to the intensity of pain on VAS, women having previous cesarean delivery experienced more severe pain before starting epidural analgesia (VAS in Groups A and C; 7.9 ± 0.2) as compared to previous vaginal delivery (VAS; Group B 6.4 ± 0.2) (P < 0.0001). The duration of second stage of labor was significantly prolonged in parturients with previous CD (Group A 22.6 ± 1.2; Group C 25.0 ± 1.9 v/s Group B 18.4 ± 1.1) (P < 0.0001). Similarly third stage of labor was also prolonged significantly in Group A and Group C (10.1 ± 0.7, 10.2 ± 0.9) as compared to Group B (7.7 ± 0.6) (P < 0.0001). However, total duration of labor was not significantly different among the three groups. (P > 0.05) Cervical dilatation on admission to the maternity ward was 4.1 ± 1.0 (Group A and C) and 4.0 ± 1.0 (Group B). The total consumption of ropivacaine in epidural analgesia was significantly high in Group A (previous CD) (29.6 ± 1.2 mg) as compared to Group B (previous vaginal delivery) (28.1 ± 1.6 mg) (P < 0.0001). Conclusions: Epidural analgesia is an effective and safe method of analgesia for vaginal delivery after previous cesarean section, and does not involve the risk of untimely diagnosis of impending uterine rupture.

Two novel web apps for W7-X are introduced: Profile Cooker and Power House. They are designed to streamline the workflow of profile fitting and power balance analysis while offering a graphical user interface that works in any common browser. This allows us to compile a comprehensive database of experimental power balance results. All fitting functions available in Profile Cooker are presented and compared on the basis of example profiles. The power balance equation assumed in Power House is established and its individual terms are discussed. The main focus of the power balance analysis is on the turbulent transport coefficients. A model for quick calculation of neutral beam power deposition based on experimental profiles is presented. Neoclassical root transition poses an issue for power balance analysis due to the uncertainty of the radial electric field. A global, neoclassical simulation with the code EUTERPE is performed for a set of experimental profiles to gain an understanding of the neoclassical root transition. [ABSTRACT FROM AUTHOR]

Larger fusion experiments require long beam paths for laser diagnostics, which requires mechanical stability and measures to deal with remaining alignment variations. At the same time, due to technical and organizational boundary conditions, calibrations become challenging. The current mid-sized experiments face the same issues, yet on a smaller scale, which makes them ideal testing environments for novel calibration methods, since a comparison with the established best practices is still possible. At the stellarator Wendelstein 7-X, the calibration and operation of the Thomson scattering diagnostic is hampered by beam displacements, coating of windows during operation, and access restrictions while the superconducting coils are active. New calibration techniques were developed to improve the profile quality and reduce calibration time. While positional variations of the laser beam have to be minimized, the remaining displacements can be accounted for during the absolute calibration. An in situ spectral calibration has been developed based on Rayleigh scattering, which calibrates the whole diagnostic, including observation windows. In addition, a less accurate but faster method has been developed, which utilizes stray-light of a tunable OPO to perform spectral calibration within minutes and does not require torus hall access. Finally, a workflow has been established to consider finite linewidths of the calibration source in the spectral calibration. While these methods will be used at W7-X to complement existing calibration techniques, they may also solve some of the aforementioned issues expected for even larger and nuclear experiments, where access restrictions are stringent and calibration becomes even more demanding. [ABSTRACT FROM AUTHOR]

A quantitative theoretical framework has been created to model neutral beam injection and fast ion losses in the Wendelstein 7-X (W7-X) stellarator, including a novel method to develop synthetic diagnostics for fast ion loss detectors (FILDs) of many types, such as scintillating and Faraday Cup FILDs. This is the first time that this has been done in stellarator geometry with this level of fidelity, providing a way for fast ion losses to be predicted more precisely in future stellarator experiments and in W7-X. Simulations of the signal seen by a Faraday Cup FILD have been completed for multiple W7-X plasmas and show close agreement with the measured signals. This method is now applied to an actively water-cooled, scintillator-based FILD, which is currently in development to measure the fast ion loss distribution in W7-X in greater detail. The design makes use of a double slit to measure energy-and-pitch-angle-resolved losses of both co-going and counter-going fast ions. The diagnostic, which can be inserted to different radial positions, has been designed to withstand steady-state heat fluxes of up to 120 kW/m2 along with additional transient heat loads of 100 kW/m2 lasting for up to 20 s at a time. Simulations of W7-X standard magnetic configuration show up to 8 × 1013 (s−1 cm−2) ion fluxes onto the sensor from each neutral beam source and no signal from the counter-going slit. These simulations will help inform experimental proposals for future W7-X campaigns after installation of this diagnostic. [ABSTRACT FROM AUTHOR]